Ultrasonic inspection is a standard method to assess the integrity of large-diameter oil pipelines. However, similar methods applied to natural-gas pipelines present a considerably greater challenge. Gas is a poor coupling agent for the probing ultrasonic signals emanating from the transducer to the pipe wall. Natural gas exhibits a very low specific acoustic impedance (300 Rayls for methane at 1 bar) compared to oil (1.5 MRayls and higher). Consequently, large ultrasonic-signal transmission losses occur at the transducer/gas and pipe-wall interfaces. To circumvent this obstacle, past exploratory developments included the use of a liquid-filled wheel, electromagnetic-acoustic-transducer (EMAT), and liquid-slug technologies. While prototypes of high-speed, in-line inspection systems employing such principles exist, all exhibit serious operational shortcomings that prevent their wide spread commercial exploitation.
Experimental results demonstrate the technical feasibility of an alternative approach to the important problem of high-speed, in-line ultrasonic inspection of natural-gas pipelines. The present invention teaches the operation of a gas-coupled ultrasonic inspection system in the classic pulse-echo configuration to detect pipeline flaws and observe wall-thickness variations. Experimental results demonstrate good signal-to-noise characteristics. Therefore, the present invention provides the enabling technology for high-speed, in-line ultrasonic inspection systems for natural-gas pipelines, risers, and similar structures. A brief summary of the related prior art follows below.
U.S. Pat No. 3,409,897 (1968) to Bosselarr et al. discloses a method for detecting and locating leaks in pipelines--mainly oil pipelines. The device detects the ultrasonic noise generated by the escape of fluid from the pipeline. This device does not address wall thickness in pipelines.
U.S. Pat. No. 3,413,653 (1968) to Wood discloses a method of detecting leaks in pipelines. The device uses a geometry of seals to detect the noise of a pipeline breach via an ultrasonic detector upstream and downstream of the pig. This method also uses a magnetic detector to detect welds. Using the three detectors it is possible to differentiate leaks, welds, and background noise from each other.
U.S. Pat. No. 3,439,527 (1969) to Rohrer discloses an apparatus to test gas mains characterized by a high pressure chamber formed by seals on a pig like device. The high pressure chamber has a microphone connected to earphones at ground level. The earphones are used to listen for the noise of leaks in the gas main. This device is not used in pipelines, and does not address wall thickness.
U.S. Pat. No. 3,478,576 (1969) to Bogle discloses a pipeline pig having an upstream and downstream detector and a delaying means to synchronize the received signals. This method accentuates the signal generated from leaks and diminishes non-point source noise.
U.S. Pat. No. 3,592,967 (1971) to Harris discloses a leak detector to detect the ultrasonic signal generated by leaks. This device is passive, not oriented to gas pipelines, and does not address wall thickness.
U.S. Pat. No. 3,810,384 (1974) to Evans discloses a device for ultrasonically measuring the wall thickness of pipelines and detecting cracks in pipelines via a pig. This device is for pipelines containing a suitable coupling fluid. Typical of such coupling fluids are the hydrocarbons (i.e. gasoline, oil, liquefied petroleum gas, or water) which surrounds the transducer and interior of the pipe wall.
U.S. Pat. No. 4,372,151 (1983) to Muraviev et al. discloses an automatic fault locating apparatus. The apparatus detects the leading edge of a pressure drop wave generated by a breach of a pipeline. This device consists of a permanently connected sensor.
U.S. Pat. No. 4,416,145 (1983) to Goodman et al. discloses a leak detector for containers as well as mechanical faults (i.e. worn bearings). The device detects the ultrasonic signals from two sources. One is the frequency shift of the signal from an ultrasonic driver. The second is from the sound made by the bursting of bubbles created by a liquid which is applied over the surface of the container after it is pressurized.
U.S. Pat. No. 4,485,668 (1984) to Hudson et al. discloses a method to detect leaks in pressurized pipes by passing a transducer through the pipe to detect leaks. The leak is detected via an above-ground receiver.
U.S. Pat. No. 4,522,063 (1985) to Ver Nooy discloses a method of detecting inadequately supported sections or overloaded points in a pipeline including the steps of traversing the interior of the pipeline with an instrumentation pig, sequentially striking or vibrating the wall of the pipeline by means carried by the pig to introduce vibratory signals into the pipeline, receiving said signals from within the pipeline by listening to the sounds generated as a consequence of the striking of the interior wall, and detecting preselected characteristics of received sound which are indicative of unsupported sections or of points of load and stress concentration in the pipeline.
U.S. Pat. No. 4,987,769 (1991) to Peacock et al. discloses a device to permit ultrasonic leak detection, especially in internal combustion engines. An ultrasonic source is housed in a tubular body adapted for attachment to a spark plug aperture in an engine and ultrasonic signals are injected into the engine cylinders. A directional ultrasonic detector is used to detect leakage signals.
U.S. Pat. No. 5,333,501 (1994) to Okada etal. discloses an abnormality monitoring apparatus that has detectors spaced at locations along a pipeline to detect sound waves. The device locates the abnormality by the time of arrival of sounds from the abnormality.
U.S. Pat. No. 3,850,028 (1974) to Thompson et al. discloses an ultrasonic electro magnetic transducer having an alternating current conductor located in the field of a permanent magnet with said conductor defining a serpentine path lying parallel to the surface of a test object to induce eddy currents in the test object flowing in directions transverse to the field of the permanent magnet. Two such transducers are provided and are employed as a transmitter-receiver pair to generate and detect Rayleigh, Lamb, or other elastic waves within the object under test without requiring contact of the transducers with the object.
U.S. Pat. No. 4,104,922 (1978) to Alers et al. discloses an electromagnetic acoustic transducer is provided for ultrasonically inspecting conductive material as the material moves relative to the transducer. A coil is positioned in the field created by a magnet so that the conductors of the coil are transverse to the magnetic field. The coil is located predominantly near the leading side of the magnet where flux is concentrated as the magnet and material move toward each other.
U.S. Pat. No. 4,092,868 (1978) to Thompson et al. discloses an electromagnetic acoustic method and device which are suitable for the in-place inspection of pipelines. A completely self-contained, mobile inspection station is placed inside a pipeline. The station runs through the pipe and transmits Lamb waves within the pipe wall, receives reflected and transmitted portions of the waves, and records the amplitude and phase of the received waves. The recorded information is analyzed to determine the location and nature of discontinuities in the pipe. This method must be used with metal pipes.
Proceedings of the 12th World Conference on Non-Destructive Testing (1989) by Boogaard et al., "Evaluation of The Techniques Implemented In Commercially Available On-Stream Pipeline Inspection Tools", summarizes the results of evaluation tests of pipeline inspection tools. This paper considers the capabilities and limitations of the available pipeline inspection techniques. The paper covers the use of ultrasonic pigs and states that "ultrasonic devices function only when a liquid surrounds the sensors". Additionally, the performance deteriorates significantly in crudes containing wax, water, or gas. This reference summarizes the state of the art in 1989.
"Gas Coupled Ultrasonics For The Inspection Of Pipes In Natural Gas Delivery Systems", (1991) a proposal to the Gas Research Institute, describes using a laser coupled transducer to generate an ultrasonic wave as practical with a gas-coupled transducer used to detect the signal. However, the proposal concluded that it would not be possible to use the same gas coupled transducer to generate and receive signals.
"Gas-Coupled Acoustic Microscopy In The Pulse-Echo Mode", (1993), C. M. Fortunko et al. demonstrates the technical feasibility of a gas coupled scanning acoustic microscope operating in the pulse-echo mode. A high pressure nitrogen or argon environment is used for this microscope. In this experiment coins encapsulated in polymethyl methacrylate (PMMA) were used as subjects. At present, 0.25 mm sub-surface lateral resolutions are attainable at 3 MHz in PMMA and even better performance should be possible at higher frequencies.
"Assessment of Technology for Detection of Stress Corrosion Cracking in Gas Pipelines" (1994), prepared for The Gas Research Institute, is an assessment of non-destructive evaluation technology that can be applied to in-line detection of stress-corrosion cracking in natural gas pipelines. The assessment revealed that no single technology has demonstrated that it meets the industry goals for such inspection, but both ultrasonics and electromagnetic methods were found to be candidates for further development. Also assessed were methods of data analysis that may be used to improve signal discrimination. Comparison tables rate the different techniques within each method and a complete bibliography is appended for related reference material.